1. Field of the Invention
This invention pertains to underground storage tanks for the storage of liquids in general, and in particular chemicals, and most particularly, fuels such as gasoline, diesel fuel, heating fuel and the like, as well as petroleum products such as machine and lubricating oils. These tanks are adapted to contain liquids underground, and dispense them, through a pump, to a distant location, such as the nozzle of a gasoline pump at a gas station. Specifically, the invention contemplates a triple walled tank, with an inner or primary wall, a secondary wall and an annulus between the primary and secondary wall, and an outer or tertiary wall, with an annulus between the second and tertiary walls.
2. Background of the Prior Art
Commercially, underground storage tanks have been in use in the United States for over one hundred years. With the development of a nationwide road system, and reliance on the automobile and related transportation, gasoline service stations were widely developed. Although not the sole site for use of underground storage tanks, typically, gasoline at a service station is held in underground storage tanks and dispensed through above-ground facilities. This invention focuses on the structure of these underground storage tanks, which can also be used to store chemicals and other liquids, but find a principal application in the storage of liquid fuels.
Initially, storage tanks were constructed of steel. The use of steel presents a problem of corrosion, however, with failure of containment likely over a long term of service. This failure became even more likely if the hole where the tank was situated was a xe2x80x9cwetxe2x80x9d hole, that is one filled with water, or typically, brine, a highly corrosive fluid. This problem spurred the development of corrosion-resistant tanks constructed of fiberglass and resin, generally referred to as fiber reinforced plastic, or FRP. Xerxes Corporation, a leader in fiberglass reinforced plastics, and its predecessor, introduced FRP underground storage tanks (storage tanks, herein) in the 1970""s, along with Owens Corning Fiberglass.
While FRP tanks provide resistance to corrosion, failure of any tank, due to mechanical weakness, impact, etc. presented the possibility of environmental contamination of the area surrounding the storage tank, as well as an expensive cleanup. A response to this concern was the introduction of the xe2x80x9cdouble walledxe2x80x9d FRP tank, in which two concentric walls, separated by an annulus in which some form of leak detection monitor was provided, were employed to provide back-up protection. Leakage in either the inner or outer wall was detected through an alarm means, or monitor, and provided the opportunity to locate and repair the damage before penetration of both walls could occur. The first such double wall FRP tank was commercially introduced by Xerxes Corporation in 1984.
Double wall storage tanks, particularly of FRP construction, have since received substantial attention in the patent literature. Among patents commonly assigned herewith are U.S. Pat. No. 5,544,974, directed to an optimized service station installation, and U.S. Pat. No. 5,595,456, directed to the provision of a water-tight riser for a double walled FRP storage tank.
U.S. Pat. Nos. 5,220,823; 4,988,447 and 4,974,739; address double walled underground storage tanks where the annular space between the inner and outer wall provides flow through of liquids therein, but provides for some strength sharing between the two walls. U.S. Pat. Nos. 5,020,358; 5,017,044; 4,825,046; 4,875,361 and 4,739,659, all commonly assigned herewith, address different solutions to the need to provide for dual containment of the interior fluid, monitoring of the integrity of the tanks, and ease of construction.
Typically, a double walled FRP storage tank is built using either xe2x80x9cmalexe2x80x9d or xe2x80x9cfemalexe2x80x9d construction. In the male method, the tanks is formed about a rotating mandrel, with a layer of FRP material formed on the outer surface of a cylindrical mandrel. Reinforcement ribs, to provide hoop strength, and resistance to buckling, are formed on the first or primary FRP layer, and adhered thereto through a secondary bond. In a double walled tank, a layer of annular material is placed over some or all of this rib bonded primary wall, and then a secondary wall of FRP material is formed on top of the annular material. Although originally, the second wall touched the tops of the ribs and spanned the distance therebetween, spaced from the region of the primary tank between the ribs, or xe2x80x9cflatsxe2x80x9d, as illustrated in U.S. Pat. Nos. 4,781,777 and 4,679,093, this created an annulus of substantial volume. As a xe2x80x9cwetxe2x80x9d annulus, that is one where the annular space is filled with a leak detecting fluid such as brine, became increasingly popular, the weight of the annular fluid in these tanks became significant, and a contour following tank was developed, where the annular material, and the secondary containment tank, follow the general contour of the primary tank ribs and flats.
In the female method, the tank is formed inside of a mold, rather than on a mandrel. The mold has ribs provided in it, giving rise to a monolithic or primary bond between the flats and the ribsxe2x80x94they are co-formed and co-cured. This gives rise to a more robust tank, with substantial structural integrity. In nearly thirty years of commercial use, not one tank of this type, manufactured by Xerxes Corporation, has failed due to lack of mechanical strength. In this system, the annular material is applied to the inside of the outer wall, and then the substantially cylindrical inner wall is formed on the inside of the annular layer. U.S. patent application Ser. No. 08/705,765, allowed, discloses this method of tank construction, where the annular material is reduced to a Mylar film, or PVA and wax in the dome shaped ends. The entire disclosure of this application is incorporated herein-by-reference.
Recently, increasing population pressure, and recognition of the sensitivity of sources of water to environmental pollution, have given rise to requests from government and private interests to provide even greater security than those offered by commercially available double walled underground storage tanks, including FRP tanks, currently available from Xerxes corporation and Containment Solutions. Enhanced, or triple wall protection, has been requested by various local governments, including those concerned about the safety of the San Antonio, Tex. aquifer, and San Francisco, Calif. Currently no triple wall tank is available, nor is a double wall tank readily available to be combined with existing technology to provide such triple wall technology.
The above-stated goals, including the provision of a triple walled tank for additional security of containment of fluids within the primary tank, is provided by combining technologies proven over time in separate tanks, so as to provide an internal or primary tank, with a secondary tank surrounding the primary tank, and an annular material spacing the two apart. A tertiary tank is placed surrounding the secondary tank, and again, an annular space is created between the secondary and tertiary tanks.
The structure can be provided in either of two embodiments. In the first embodiment, the primary and secondary tank, with the annulus there between, are built in a fashion identical to that disclosed in U.S. patent application Ser. No. 08/705,765. Specifically, the outer or secondary tank is formed on the interior of a female mold, with a mylar film or other separating material applied along the flats of the interior of the secondary tank. A layer of unidirectional fabric, or xe2x80x9cunixe2x80x9d is placed over the open ends of the ribs formed in the secondary tank, and coated with FRP materials, to seal the rib. In at least three points around the circumference of the tank, xe2x80x9cguttersxe2x80x9d of dimensionally stable material which permits the flow of liquid longitudinally are installed, connecting each annular space and the space within each rib, by punching holes in the overlaying material. Thereafter, the interior, or primary tank of FRP materials is xe2x80x9csprayed upxe2x80x9d, or formed on the interior of the annular material. Each tank is formed from two half-shells, each with a closed, dome-shaped end. The primary and secondary tanks are separated, in the dome, by application of a polyvinyl alcohol (PVA)/wax combination, insuring the flow of liquid between these walls in the dome. The two half shells are then married, and secured by an overlay of FRP material on either side of the joint.
In a first embodiment, a tertiary wall is provided with an annulus between the secondary wall and itself, by forming a smooth cylinder which rests on, and in preferred embodiments, is secured, to the tops of the ribs of the secondary containment wall. This tertiary wall is preferably formed by wrapping Bayex(copyright) or similar FRP supporting fabric about the exterior of the tank, from rib top to rib top, and then xe2x80x9cspraying overxe2x80x9d or applying resin or fiber reenforced plastic there over, and allowing the same to cure. The tertiary wall defines annular chambers between each adjacent pair of ribs. To provide for fluid communication between the chambers, an insert is provided at one or two locations along the rib, on the top of the rib, providing for flow between adjacent annular chambers.
In an alternate embodiment, the double walled half shells, with an annulus there between, are not removed from the mold. In this embodiment, the outer most wall formed against the mold interior surface becomes the tertiary wall, and the wall previously formed becomes the secondary wall. A second annular material is applied to the inner surface of the secondary wall, and secured thereto, typically by providing resin to adhere the annular material to the interior wall by providing a tacky surface. Against the interior of this second annulus, a primary wall is built, creating a half-cylinder in each mold half. Two mold halves are then married to form a single tank.
In either embodiment, the reenforcing ribs, necessary for buckling resistance and hoop strength, are integrally formed with the wall with which they are associated (either the secondary or tertiary wall, in the first or second embodiments, respectively). This provides for a strong, robust and durable tank. In preferred embodiments, there is a bond between each of the walls, either directly, or through a strength-sharing, load transmitting annular material. Each annulus is provided with a leak detection system, so that leakage through any of the three walls can be detected quickly, and repaired without loss of containment.